Directional manipulation of diffusio-osmosis flow by design of solute-wall and solvent-wall interactions

Understanding of diffusio-osmosis, the flow induced by a solute gradient acting in narrow interfacial layers at a nanoscale solid-liquid interface, is of great value in view of the increasing importance of micro- A nd nano-fluidic devices and self-propelling particles. Using molecular dynamics simulations, we employ an appropriate strategy for direct simulation of diffusio-osmosis flows, mimicking a realistic experiment without any assumed external forces. It allows us to obtain reliable flow details, which are hard to obtain in experiments. We found that the solvent-wall interaction, previously overlooked in the classical paradigm, plays a critical role in the diffusio-osmosis process. In particular, diffusio-osmosis is controlled by the interaction difference between the solute-wall and solvent-wall. When the solute-wall interaction is stronger (weaker) than the solvent-wall, a surface excess (depletion) of solute particles on the solid-liquid interface is formed, which induces diffusio-osmosis flow towards a low (high) concentration. We modified the classical Derjaguin expression to include the effect of nanoscale hydrodynamics boundary conditions for the accurate prediction of diffusio-osmosis characteristics. Overall, our results provide clear guidance for controlling fluid flow and manipulating the motion of colloids under tunable solute concentrations.